Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a trace gas, which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and is measured. The quantity of helium corresponds to the leak rate. An important component of the instrument is a mass spectrometer, which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer in order to separate the helium component, which is then measured. In one approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector.
One of the requirements of the mass spectrometer is that the inlet through which the helium and other gases are received be maintained at a relatively low pressure, typically below 2×10−4 Torr. In a conventional leak detector, a vacuum pumping arrangement is utilized to maintain the input of the mass spectrometer at the required pressure. However, since the test port must be maintained at a relatively low pressure during a leak test, the rough pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, which results in a high test port pressure, it may be difficult or impossible to achieve the required pressure level.
In a counterflow leak detector disclosed in the U.S. Pat. No. 3,690,151, issued Sep. 12, 1972 to Briggs, the mass spectrometer tube is connected to the inlet of a diffusion pump and the helium trace gas is introduced through the foreline, or exhaust port, of the diffusion pump. The diffusion pump exhibits a high pressure ratio for heavier gases, but a low pressure ratio for lighter gases such as helium. Therefore, helium diffuses at an acceptable rate in a reverse direction through the diffusion pump to the mass spectrometer and is measured. Heavier gases in the sample are to a large degree blocked by the diffusion pump and prevented from reaching the mass spectrometer. Due to the method of reverse flow in the diffusion pump, the leak detector test port can be operated at a much higher operating pressure, typically 100 millitorr.
A test port pressure of 100 millitorr is satisfactory for many leak test applications. However, it is desirable in some applications to perform leak tests on very large or leaky parts where this test port pressure cannot be attained. In another prior art arrangement, a flow restrictor is positioned between the test port and the foreline of the high vacuum pump. Separate roughing pumps are used to pump the test port and the foreline of the high vacuum pump. This approach permits a higher test port pressure but is more complex and expensive because of the need for two roughing pumps.
Another prior art approach is disclosed in the U.S. Pat. No. 4,735,084, issued Apr. 5, 1998, to Fruzzetti. The trace gas passes in reverse direction through one or two stages of a mechanical vacuum pump, thereby achieving a high test port pressure.
A counterflow leak detector with high and low sensitivity operating modes is disclosed in the U.S. Pat. No. 4,845,360, issued Jul. 4, 1989, to Landfors. A diffusion pump includes a conventional foreline and a second foreline provided with an ejector stage. The leak detector has high and low sensitivity operating modes.
A leak detector which utilizes a turbomolecular vacuum pump having an inlet connected to a gas sensor, an outlet connected to a forepump and an intermediate inlet connected to the test port is disclosed in the U.S. Pat. No. 4,472,962, issued Sep. 25, 1984 to Mennenga.
The U.S. Pat. No. 5,542,828, issued Aug. 6, 1996, to Grenci et al. discloses a system for vacuum pumping a mass spectrometer, which uses a scroll vacuum pump in combination with a high vacuum pump.
None of the prior art arrangements for leak detection of large volume and/or leaky parts at relatively high pressures has been entirely satisfactory. Accordingly, there is a need for new and improved systems and methods for trace gas leak detection of large leaks at relatively high test pressures.